In the production of seamless tubing, for example, a finite section of pierced tubing is processed in a stretch reducing rolling mill, in order to reduce the diameter of the tubing to a predetermined size. In a stretch reducing mill, the tubing is also elongated under tension during the rolling process, in order to control the wall thickness of the tubing. In a typical stretch reducing mill, there may be as many as 24 mill stands, for example, arranged in a close coupled sequence. The Gillet U.S. Pat. No. 3,355,923 is illustrative of the physical arrangement of a typical stretch reducing mill.
When a pierced tubular workpiece enters the successive passes of a stretch reducing mill, it is successively reduced in diameter. This of course results in elongation of the tubing, such that successive mill stands are driven at progressively higher speeds to accommodate the progressively lengthening work. In addition, in order to control the wall thickness of the tubing, it is desired to further elongate the tubing under tension between mill stands. The generalities of these procedures are, of course, well known in the industry.
As may be understood, a given intermediate area of a tubular workpiece passing through a multi-stand mill is influenced by all of the mill stands, both upstream and downstream from the mill stand through which the given area is passing. Thus, a section of tubing in the twelfth stand of a 24 stand mill is influenced by the relative retarding action of all of the upstream mill stands and the relative pulling action of all of the downstream mill stands, and this combined influence is reflected in processing of the tube at the twelfth mill stand. However, when the head end of the tubing first enters the mill, there can of course be no influence deriving from mill stands in the downstream portions of the mill at which the tubing has not yet arrived. Likewise, as the tail end of the tubing section passes through the mill, there is no influence derived from the empty upstream mill stands. As a result, the stretching effect achieved in the head end and tail end portions of a finite length of tubing is significantly less than in the central portion, tending to result in off-specification product in the head end and tail end areas.
Customarily, the off-specification end areas are cropped off and scrapped. As is readily apparent, the shorter the overall length of tubing, the greater is the percentage loss represented by the crop ends. Especially in connection with seamless tubing, where the tubing sections are relatively short in order to be driven over a piercing mandrel of acceptable length, the crop end losses can represent an undesirably high percentage of the overall tonnage.
The problem of overall tension control in the head end and tail end portions of rolled metal products has been recognized for some time, and various efforts have been made to effect a reduction in the crop losses of such products. Among such prior proposals is that of U.S. Pat. No. 3,645,121, in which progressive speed variation in successive mill stands is disclosed. However, the procedure of this patent is not workable in a practical way, and does not recognize the fundamental considerations involved. British Patent Specification No. 1,274,698, also discloses the generalities of a procedure for controlling the speed of stretch reducing mills to reduce head end and tail end crop losses. As in the case of the beforementioned United States Patent, however, the generalities of the disclosed process are crude and lack specificity, such that only limited advantages are realized. The Hayashi U.S. Pat. No. 3,874,211 utilizes a combination of tension and screw-down control to minimize crop end loss in tube rolling. Similar practices have been followed in the rolling of metal strip, as for example reflected in the Stoltz U.S. Pat. No. 2,281,083, and Stringer U.S. Pat. No. 3,110,203 where back tension and forward tension on the strip is controlled to reduce off-specification material at the head end and tail end of a finite strip. In the Wallace U.S. Pat. No. 2,972,268, a combination of screw-down and tension control is provided.
While the prior art adequately discloses the generality of tension control for minimizing head end and tail end crop loss, less than optimum effectiveness has been achieved in the end result. The procedures of the present invention serve to optimize head end and tail end rolling procedures, particularly for the stretch reducing rolling of tubing, to provide a greater yield of specification material over the length of the tubing blank as compared to prior art techniques for achieving crop loss reduction.
Pursuant to the invention, a multiple stand stretch reducing mill for seamless tubing and the like (e.g., electric weld or other tubing which is heated prior to stretch reducing) is controlled according to predetermined calculation for tubing of given physical and metallurgical characteristics, whereby the processing of the head end and tail end sections of the tubing can be carried out within specification over a greater length than has been practicable heretofore in commercial scale operations. The procedure of the invention involves in part the determination for a tubing section of given physical and metallurgical characteristics at a given mill stand, of the maximum driving forces that may be applied thereto by a given mill stand, without excessive slippage between the mill rolls and the workpiece. In addition, the process involves a determination for a tubing section of given size, wall thickness, metallurgical characteristics, temperature, etc. of a predetermined maximum stretch factor, beyond which detrimental yielding of the material might be experienced. These calculated parameters are applied to the operation of the mill stands in such a way that maximum driving forces may be applied to the end sections of the workpiece, for maximum elongation of the end sections, while at the same time the predetermined maximum stretch factor is not exceeded in any case.
In the processing of leading or head end portions of a tubular workpiece, the procedure of the invention involves the variable control of upstream mill stands, as the head end proceeds into the stretch reducing mill. Initially, the mill stands are operating at a predetermined, steady-state speed. As the head end enters, successive mill stands are decelerated according to a pre-calculated program, such that, whenever the head end is engaged in three or more mill stands, two of the mill stands are exerting maximum driving force, one in the pulling direction and one in the restraining direction, while an intermediate mill stand is driven to establish a predetermined equilibrium of pulling forces on either side of it. In any case where the exertion of maximum pulling and restraining forces by programmed mill stands is such as to tend to exceed the maximum stretch factor of the tubing in the intermediate tubing section, the mill speed program according to the invention provides for a plurality of intermediate mill stands, each programmed to exert less than maximum driving force on the tubing, and calculated to maintain substantial force equilibrium on opposite sides of each of the intermediate mill stands, and also serving to maintain the stretch factor in any area of the intermediate tubing section at or below the predetermined maximum stretch factor for the physical and metallurgical characteristics of the tubing at that stage of the process. The procedures of the invention recognize that the character of the workpiece is changing as it progresses through the mill, and the pre-calculated mill stand speeds are determined in such a manner that effective tensions applied to the head end and tail end sections of the tubing are limited primarily by the ability of the mill stands to apply driving force without excessive slippage, or by the limiting stretch factor.
Whereas prior art proposals for limiting crop end loss largely are concerned with the progressive acceleration or deceleration of successive mill stands for applying progressively increasing tensions, the procedures of the invention, recognizing the important basic parameters to be observed, achieve optimum reduction of crop end loss by mill speed control which is not necessarily progressive. Rather, more typically, there is a wave characteristic to mill speed control of the variable speed mill stands when following the procedures of the invention.
In a typical application of the process of the invention, a finite length of tubing is processed in a multi-stand stretch reducing mill, which may contain, for example, as many as twenty-four successive mill stands. Pursuant to the invention, while it is theoretically possible to provide individual, independently variable speed control for each of the twenty-four mill stands, in such a mill, there generally is little practical economical justification for providing independent variable control for that many mill stands. More typically, the objectives of the invention may be largely satisfied in a mill installation of reasonable cost, by providing for the necessary independent variable speed control in the first eight or ten mill stands.
For a more complete understanding of the procedures of the invention, reference should be made to the following detailed description of preferred embodiments thereof, in conjunction with the accompanying drawings.